1. Field of the Invention
The present invention relates to an oscillation circuit using an oscillator composed of a quartz oscillator or a ceramic oscillator as an oscillation source.
2. Description of the Related Art
FIG. 1 is a circuit diagram showing a conventional oscillation circuit using an oscillator. An oscillator 81 which is composed of a quartz oscillator or a ceramic oscillator, an inverter 82 and a feedback resistor 83 are connected in parallel. A capacitor 84 is connected between the earth voltage Vss and one terminal of the oscillator 81 and a capacitor 85 is also connected between the earth voltage Vss and the other terminal of the oscillator 81.
In general, with the above oscillation circuit, an oscillation start time is shortened and an oscillation start voltage is lowered if a large value of mutual conductance gm of the inverter 82 is set. As a result, the inverter 82 can be operated by a low voltage, for example, almost 3 V. However, a penetrating current, i.e., a current which flows through the inverter 82 from the power source voltage Vcc to the earth voltage Vss is increased when the oscillation circuit is in oscillation operation near the operating point so that it is difficult to obtain a low consumption current with the above oscillation circuit.
As shown in FIG. 2, another conventional oscillation circuit in that a clocked inverter 86 is connected in parallel with the inverter 82 has been developed. In the oscillation circuit in FIG. 2, both the inverter 82 and the clocked inverter 86 are operated to cause the oscillation until the oscillation begins. The clocked inverter 86 is turned off after a predetermined time period elapsed from the beginning of the oscillation, and thereafter the oscillation operation is maintained only by the inverter 82. Thus both the short oscillation start time and the low consumption current can be realized by the above circuit.
However, when the oscillation circuit is operated by the low voltage such as almost 3 V, the operating point of oscillation of the inverter 82 is (1/2)Vcc, i.e., 1.5 V where Vcc is the power source voltage. Generally, when the inverter 82 is embodied by means of the CMOS technique, it is constructed by connecting serially both the source-drain connections of P-channel MOS transistor and N-channel MOS transistor between the power source voltage Vcc and the earth voltage Vss, as shown in FIG. 3. A threshold voltage Vth of the MOS transistor is a typical parameter which has an influence upon the characteristics of both MOS transistors. The absolute value of the threshold voltage Vth is ordinarily set at about 1 V. At that time, the current Icc flowing at the operating point of the CMOS inverter is given by following proportional expression as EQU Icc.varies.(V.sub.GSN -V.sub.thN).sup.2 ( 1)
where V.sub.GSN is a gate-source voltage of the N-channel MOS transistor shown in FIG. 3, and V.sub.thN is a threshold voltage of the same.
However, a difference between the gate-source voltage V.sub.GSN and the threshold voltage VthN is reduced when the power source voltage Vcc is lowered. As a result, the variation of the current Icc is increased due to the variations of the power source voltage Vcc and the threshold voltage V.sub.thN. For example, when V.sub.thN =0.9.+-.0.3 V and Vcc=3.+-.0.3 V, a ratio of maximum value Icc(MAX) and minimum value Icc(MIN) is given by following equation. ##EQU1##
There is a difference of 49 times between the maximum and the minimum values of the operating currents. Assume that the operating current Icc required to maintain the oscillation is, for example, 100 .mu.A, so much current of 4.9 mA flows at maximum owing to the variations of the power source voltage Vcc and the threshold voltage V.sub.thN.
Accordingly, even if it is tried to lower the consumption current with the oscillation circuit in FIG. 2, the variations of the operating currents are increased when the circuit is operated by the low power source voltage and thus the low consumption current cannot be attained by this circuit. In other words, the oscillation circuit of FIG. 2 can not only shorten he oscillation start time, but also lower the oscillation start voltage. However, it cannot lower the consumption current under the low power source voltage.